Diamond coatings deposited on WC-Co substrate by multiple laser

ABSTRACT

A diamond coating formed on a WC-Co substrate prepared through a process including employing a plasma and a variety of interactions from a multiple laser system demonstrates exceptional adhesion and indicates a durable cubic diamond structure. The coating on the WC-Co substrate is typically between 25 and 40 mum thick and has an average crystal size of between 10 and 20 mum. Various methods of confirming the cubic diamond structure of the coatings have been employed. The adhesion of the diamond coating to the substrate is very strong. An electron microprobe analysis shows tungsten and cobalt atoms incorporated into the film and a layer depleted in cobalt exists at the diamond-WC-Co interface. Particulates of WC-Co-C alloy are spread over the top surface, apparently formed by condensation from the vapor phase of metal-containing molecules. Carbon is confirmed as being the main component of the surface layer.

This application claims priority from provisional application Ser. No.60/093452, filed Jul. 20, 1998.

TECHNICAL FIELD

The present invention is generally directed to diamond coatings. Moreparticularly, the present invention is directed to diamond formation ona WC—Co (tungsten carbide-cobalt-carbon) substrate using multiplelasers.

BACKGROUND ART

The concept that diamond can be grown exclusively within itsthermodynamic stability region is no longer valid. (Reference may be hadto, for example, A. Badzian, T. Badzian, Int. J. of Refractory Metalsand Hard Materials 15 (1997) 3.) The search for novel approaches todiamond synthesis, different from HP/HT and CVD operations, continuesdespite opinions that these two classical methods are sufficient formost applications. The new research is surrounded by the uncertainty inthe growth mechanisms. This is the situation with the recentdemonstrations of a laser induced process conducted in the open air(see, for example, P. Mistry, M. C. Turchan, S. Liu, G. O. Granse, T.Baurman, M. G. Shara, Innovations in Materials Research 1 (1996)193) andon hydrothermal growth (see, for example, X-Z Zhao, R. Roy, K. A.Cherian, A. Badzian, Nature 285 (1997) 513). The chemical reaction pathsare unknown for these two new processes, and an explanation does notseem to be forthcoming. Diamond coatings on WC—Co cutting tool insertsby the laser process are factual and have been successfully tested forwear resistance. Nevertheless, a plausible growth hypothesis has yet tobe presented.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to define a methodof adhesion of a diamond coating on a substrate.

It is a further object of the present invention to define such a methodas it applies between the diamond coating and a WC—Co substrate.

Still a further object of the present invention is apply the definedmethod to the requirements of electron field emission.

The process of applying a diamond coating to a WC—Co substrate toprepare includes the steps of employing a plasma and a variety ofinteractions from a multiple laser system using WC—Co substrates. Theprocess is conducted in open air and does not involve hydrogen.Structural characterization of the diamond coatings, which haveexceptional adhesion to cutting tool inserts, indicates a cubic diamondstructure.

The coatings on the WC—Co substrate are typically between 25 and 40 μmthick. The average crystal size is between 10 and 20 μm. Various methodsof confirming the cubic diamond structure of the coatings have beenemployed. The adhesion of the diamond coating to the substrate is verystrong. An electron microprobe analysis shows tungsten and cobalt atomsincorporated into the film and a layer depleted in cobalt exists at thediamond-WC—Co interface. Particulates of W/Co/C alloy are spread overthe top surface, apparently formed by condensation from the vapor phaseof metal-containing molecules. Carbon is confirmed as being the maincomponent of the surface layer.

Electron field emission current densities, useful for flat paneldisplays of 6 mA/cm² at an applied voltage of 3000 V for a film-anodedistance of 20 μm has been measured.

BRIEF DESCRIPTION OF THE FIGURES

The present invention will be more fully understood by reference to thefollowing detailed description of the preferred embodiments of thepresent invention when read in conjunction with the accompanyingdrawings, in which:

FIG. 1a is a schematic illustration of the process according to thepresent invention;

FIG. 1b is a plan view of a sample preparation station for fabricatingthe film-coated sample used for investigation in the present invention;

FIG. 2 is a secondary electron image of a cross-section of diamondcoating on WC—Co insert with the bar corresponding to 10 μm;

FIG. 3 discloses a pair of side-by-side X-ray maps taken oncross-section as shown in FIG. 2 by electron microprobe with the leftimage mapping Co(K∝) distribution and the right mapping W(M∝)distribution and showing depletion in Co down to 7 μm below thediamond-WC—Co interface;

FIG. 4 is a valence band spectra taken with MgK∝ and specificallyshowing CVD disordered diamond grown from CH₄—H₂—N₂—O₂ microwave plasma,the sample diamond as grown, and the sample diamond annealed in H₂plasma;

FIG. 5 is a schematic of an electron field emission measurement systemwith the anode being five times smaller than diamond coatings;

FIG. 6 is a comparison of I-V curves for the sample coating anddisordered tetrahedral carbon (DTC) grown from CH₄—H₂—N₂—O₂ plasma;

FIG. 7 is a comparison of Fowler-Nordheim plots for I-V shown in FIG. 6;and

FIG. 8 is a logarithmic plot of I-V data.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The drawings disclose the preferred embodiment of the present invention.While the configurations according to the illustrated embodiment arepreferred, it is envisioned that alternate configurations of the presentinvention may be adopted without deviating from the invention asportrayed. The preferred embodiment is discussed hereafter.

The Process And Preparation Of A Sample

Laser assisted growth of diamond was discovered by one of the inventorsherein during experiments on the surface modification of metal alloys.When CO₂ gas was involved, by a fortunate set of laser processparameters, a new type of deposit was recognized and was soon identifiedas diamond. (For a discussion on the actual discovery, see P. Mistry, M.C. Turchan, S. Liu, G. O. Granse, T. Baurman, M. G. Shara, Innovationsin Materials Research 1 (1996)193).

FIG. 1a is a schematic illustration of the process according to thepresent invention. The figure generally illustrates the lasers and gaseswith respect to the substrate. FIG. 1b discloses a plan view of a samplepreparation station, generally illustrated as 10, which was establishedfor the preparation of a substrate for study and use. A substratesupport 12 is centrally disposed surrounded by an array of lasers. Thelasers of choice include two excimer lasers 14, 14′, one YAG:Nd laser16, and one CO₂ laser 18. An alternate array of lasers having adifferent composition may be substituted for the described array,provided substantially the same results are achieved. However, theseparticular lasers were selected after due experimentation as havingsufficient power to ignite the plasma which engulfs the substrate as setforth hereinafter.

A CPU 20 is provided for selectively operating the lasers 14, 14′, 16,and 18 according to a prepared program. The excimer lasers 14, 14′ areoperatively associated with the CPU 20 via a pair of cables 22, 22′. TheYAG:Nd laser 16 is operatively associated with the CPU 20 via a cable24. The CO₂ laser 18 is operatively associated with the CPU 20 via acable 26.

The station is established in an open air environment. (This was foundto work for the selected WC—Co substrate material.) An array of gas jets28, 30 is also disposed around the support 12. The gas jet 28 isoperatively associated with a source of N₂ gas 32 via a gas conduit 34.The gas jet 30 is operatively associated with a source of CO₂ gas 36 viaa gas conduit 38.

A sample substrate (not shown) was selected and placed on the substratesupport 12. The selected samples were WC—Co inserts for cutting toolsgenerally having widths of about 15 mm although it is to be understoodthat a variety of sample substrates could be selected. Once the samplesubstrate was placed on the support 12, selected quantities of the N₂and CO₂ gases were delivered to shroud the substrate radially. Nohydrogen is involved. The focused lasers 14, 14′ 16, and 18 were thenactivated and were are operated in their pulsing modes as directed bythe program of the CPU 20. The lasers 14, 14′, 16, and 18 were guided bydrive motors (not shown) to move across the substrate surface in aprogrammed manner as dictated by the CPU 20.

During the operation, a luminous plasma is created a few mm above thesurface of the substrate which itself undergoes surface melting. Thesequence of the laser pulse width and pulse frequency are adjusted aselements of the process parameters. The duration of the depositionprocess is generally about 40 s, and this length of time was employed inthe preparation of the test sample. (Normally it takes about 40 s tocoat the WC—Co insert with an area of 1.5 cm².) During this time thegrowth rate approaches 1 μm/s and the resulting diamond film was shownto have a thicknesses of between 20 and 40 μm. Of course, it is to beunderstood that variations of the parameters including laser time andpulse rate effect variations of the growth rate and resulting filmthickness. A more complete discussion of the process and its history maybe found in co-pending U.S. Ser. No. 09/357,621, filed concurrent withthe present application.

This represents the first time four lasers have been applied in materialsynthesis. The described multiple laser process connects manyinteractions including photothermal and photolytic processes (see, ingeneral, J. G. Eden, Photochemical Vapor Deposition, John Wiley andSons, New York, 1992, pp. 5-46) and the formation of shock wave frontsat the surface.

Analysis of Resulting Diamond Coatings

A sample prepared using the above process is shown in FIG. 2 which is asecondary electron image of a cross-section of diamond coating on WC—Coinsert with the bar corresponding to 10 μm. This image reveals a diamondcoating on a WC—Co substrate which is consistent with other experimentalfindings which disclose that the coatings are typically 25-40 μm thickwith average crystal grain size of 10-20 μm. Confirmation of the cubicdiamond structure of these coatings is supported by the following data:

(1) Chemical analysis indicates carbon as the main component. Theconcentration of W in the film has been estimated as 0.2 wt %. Coconcentration is lower. Electron microprobe showed that the W and Coconcentrations were constant across the measured cross-section (i.e.,from the interface to the top of the film). The concentrations of 0 andN are at the detectability limit of X-ray Photoelectron Spectroscopy(XPS) and Auger Electron Spectroscopy (AES).

(2) X-ray diffraction (CuKα) indicates 111, 220, 311, 400 and 331 cubicdiamond lines, they fit to lattice constant of α=0.3567±0.0001 nm. Thecoatings differ in the degree of long range order.

(3) The position of the diamond Raman peak is shifted to 1336 cm⁻¹,indicating stress in the film. The narrowest line full width at halfmaximum was 6.4 cm⁻¹ and the peak to background ratio was 2.7. All filmsshow luminescence.

(For a more detailed analysis of such analysis, see A. Badzian, R. Roy,P. Mistry, M. C. Turchan, in: A. Paoletti, A. Tucciarone (Eds.) ThePhysics of Diamond, IOS Press, Amsterdam, 1997.)

The films prepared according to the above-described process differ fromconventional CVD films in many respects. The sample coatings arecontaminated by W and Co and have particulates (˜1 μm in size) on thetop surface composed of W, CO and C. Despite some similarity inphotoluminescence, they differ in many other signatures. Coatingmorphology has a non-columnar microstructure which also differs from CVDfilms.

The Diamond-WC—Co Interface

Adhesion of diamond coatings to the WC—Co inserts was found to be verystrong. The resulting sample was tested for adhesion using a RockwellSuperficial Hardness tester. Three indents were made on the specimenusing a Brale-C type indenter. A load of 45 kg was applied. An opticalmicroscope was used to determine if any delamination or cracking hadoccurred. No cracking or delamination was observed for the threeindents. A small rupture, where the indenter and the sample came intocontact, was the only evidence that the load was applied.

The sample inserts have been tested for wear resistance by an outsideservice supplier and were found to be slightly inferior topolycrystalline diamond compacts but much better than CVD coatings. Thisresult is consistent with the chemical composition of the WC—Co alloy atthe interface. An electron microprobe analysis shows a layer in theWC—Co substrate depleted of Co. This is illustrated in FIG. 3. Withparticular reference to that figure, a pair of side-by-side X-ray mapstaken on cross-section with a microprobe are shown. The left image mapsCo(K∝) distribution. The right image maps W(M∝) distribution. Thedepletion in Co is seen down to 7 μm below the diamond-WC—Co interface.Removing some of the Co from intergranular regions, before diamondnucleation begins, creates a strong bonding condition for the diamond,which anchors between the WC grains. Were this not the case, a roughinterface would be created between the metal and insulator.

The Upper Surface of The Coating

The sample disclosed that particulates of WC—Co—C (tungstencarbide-cobalt-carbon) alloy are spread over the top surface. It isbelieved that they were formed by condensation from the vapor phase ofmetal-containing molecules.

XPS spectra taken from the top surface confirm carbon as the maincomponent. The valence band spectra show some differences from CVDdiamond. The CVD diamond sample was prepared by microwave plasma CVD,with N₂ and O₂ additions, at P=80 Torr and at a substrate temperature of950° C. (See B. L. Weiss, A. Badzian, L. Pilione, T. Badzian, W. Drawl,Appl. Phys. Lett. 7 (1997) 794.) Comparisons of the spectra are shown inFIG. 4 which illustrates valence band spectra taken with MgK∝. FIG. 4also shows CVD disordered diamond grown from CH₄—H₂—N₂—O₂ microwaveplasma and a comparison between the sample diamond as grown and thesample diamond annealed in microwave H₂ plasma for 15 min at 950° C. Thefeature at 26 eV can be related to O and the peak at 32 eV to W/C alloy.

Electron Field Emission

A considerable amount of research has been reported on field emissionfrom diamond. Recent studies have shown low turn on fields (˜1 V/μm) forelectron emission devices. (Reference may be had to J. W. Geis, J. C.Twitchell, T. M. Lyszczarz, J. Vac. Sci. Technol. B14 (1996) 2060 and K.Okano, S. Koizumi, S. Ravi, P. Silva, G. A. J. Amaratunga, Nature 381(1996) 140.) Cold cathode applications require materials which show aproper combination of dielectric strength and electrical carriertransport.

The samples prepared following the above-stated method, with acombination of exceptional adherence of diamond film to the substrateand anticipated doping of diamond with W and Co atoms, can facilitateelectron field emission. A more complete discussion of the applicationof the present method for uses in electron field emission may be foundin co-pending U.S. Ser. No. 91/357,622, filed concurrent with thepresent application.

The electron emission measurements have been performed using the systemshown in FIG. 5 in a vacuum of 10⁻⁷ Torr. The anode-diamond surfacespacing was set in the range of 20-50 μm. The anode cross-section is0.33 cm² and diamond surface area is 1.5 cm².

The rough metal-insulator interface can cause an enhancement of electroninjection into the diamond conduction band by internal field emission.Specifically, the 1 μm WC crystals cause a field enhancement at themetal-diamond interface. The top surface of the diamond film alsoenhances electron emission due to the sharp edges of the diamondcrystals (refer back to FIG. 2), which cause a concentration of theelectrical field lines.

The I-V curves of FIG. 6, the Fowler-Nordheim (FN) plots of FIG. 7 andthe logarithmic plots of FIG. 8 show a comparison of the sample diamondarticle prepared according to the above process and the CVD diamond.More particularly, FIG. 6 illustrates a comparison of I-V curves for thesample coating and disordered tetrahedral carbon (DTC) grown fromCH₄—H₂—N₂—O₂ plasma, FIG. 7 is a comparison of Fowler-Nordheim plots forI-V shown in FIG. 6, and FIG. 8 is a logarithmic plot of the generatedI-V data.

The CVD diamond films were grown from a microwave plasma with N₂ and O₂additions to the CH₄/H₂ mixture. Under such conditions, the tetrahedralnetwork of carbon atoms is distorted and this is regarded by theinventors herein as diamond disordered tetrahedral carbon. The I-Vcurves indicate similar turn-on voltages for both samples, however, theemission current is noticeably different. The FN plots further highlightthis difference. The sample coatings appear to have the requirements fora cold cathode material. The films had an electrical resistance in therange of MΩ. This can be attributed to the presence of metals in thefilm. The maximum applied voltage was 3000 V, for a ˜20 μm film-anodespacing.

A cautionary statement about electric field calculations should be made.It would be convenient to determine the electric field from F=(AppliedVoltage)/(film-anode spacing). However, the potential drop across thefilm, which determines the breakdown field, and the vacuum field, whichdetermines the tunnelling barrier, are not known. These observations aresignificant when interpreting the I-V and FN characteristics.

It is speculated that the incorporation of W and Co and perhaps N and O,together with the formation of associated lattice defects, connected toobserved photoluminescence, can affect the electronic band structure ofdiamond and create additional localized electronic states in the bandgap. These new states can facilitate injection of electrons into theconduction band of diamond and undergo quasiballistic transport throughthe film. As such, ballistic transport can be a limiting step forelectron emission. Also, the negative electron affinity property ofdiamond is of importance, however, electrons still need to be injectedinto the conduction band and transported to the surface to exploit theNEA.

Cutler et al. (P. H. Cutler, Z. H. Huang, N. M. Miskovsky, P. D.Ambrosio, N. M. Chung, J. Vac. Sci. Technol. B14 (1996) 2020) usingMonte Carlo (MC) simulation demonstrated that diamond does exhibitballistic or quasi-ballistic behavior. The nature of the transportdepends upon the electric field, film thickness and the types ofinteractions experienced by the electron.

Fitting et al. (H. J. Fitting, J. Boyde, J. Reinhardt, Phys. Stat. Sol.(1) 81 (1984) 323 and H. J. Fitting, A. Von Czarnowski, Phys. Stat. Sol.(a) 93 (1986) 385) performed MC simulations along with experimentalelectron energy distributions on II-VI materials. These studies alsoshowed that the electrons exhibit a ballistic type transport and weretermed ‘hot’ electrons (i.e., they had energies greater than theconduction minimum). Recently, Fitting et al. (H. J. Fitting, Th.Hingst, E. Schreiber, E. Beib, J. Vac. Sci. Technol. B14 (1996) 2087)have reported upon vacuum emission of hot electrons in GaAs.

Summary

The multiple laser assisted diamond growth process set forth and studiedherein provides useful coatings for a variety of applications such astool inserts because of the excellent adhesion characteristics. Thefollowing properties of these coatings were helpful in conductingelectron field emission experiments:

adhesion of diamond to WC—Co substrate

electrical conductivity of diamond coating caused by doping with W andCo atoms

rough interface between WC—Co substrate and diamond coating

rough top surface of diamond.

The emission current measured at 3000 V and film-anode distance of 20 μmreached 6 mA cm⁻² justifying application across a broad range of uses.

Those skilled in the art can now appreciate from the foregoingdescription that the broad teachings of the present invention can beimplemented in a variety of forms. Therefore, while this invention hasbeen described in connection with particular examples thereof, the truescope of the invention should not be so limited since othermodifications will become apparent to the skilled practitioner upon astudy of the drawings, specification and following claims.

What is claimed is:
 1. A coated substrate comprising: a substrate; adiamond coating on said substrate; and an interface between saidsubstrate and said diamond coating, said coating being between 25 and 40μm thick and being defined by crystals, the average size of each of saidcrystals being between 10 and 20 μm, said coating including tungsten andcobalt atoms incorporated into said coating, said coating including alayer depleted in cobalt approximately at sald diamond-WC—Co interface.2. The coated substrate of claim 1, wherein said coating includes a topsurface and wherein said top surface includes particulates of WC—Co—Calloy spread thereover.
 3. The coated substrate of claim 1, wherein saidcoating mainly comprises carbon.
 4. The coated substrate of claim 1,wherein the depletion of Co appears down to 7 μm below the diamond-WC—Cointerface.
 5. The coated substrate of claim 1, wherein said substrate issubstantially composed of WC—Co interface.
 6. The coated substrate ofclaim 5, wherein the interface between the said diamond coating and saidWC—Co substrate is unsmooth.
 7. A system for diamond coating asubstrate, the system comprising: a substrate support; an array oflasers situated about said substrate support, said array of lasers beingselected from the group consisting of eximer, YAG:Nd, and CO₂ lasers; acentral processing unit operation associated with said array of lasersfor controlling the operation of said lasers; and an array of gas jetssituated about said substrate support to deliver gas to assist incoating the substrate.
 8. The system for diamond coats a substrate ofclaim 7, further indicates a source of N₂ gas, said source beingconnected to at least one gas jet of said array of gas jets.
 9. Thesystem for diamond coating a substrate of claim 7, further including asource of CO₂ gas, said source being connected to at least one gas jetof said array of gas jets.
 10. The system for diamond coating asubstrate of claim 7, wherein said array of lasers includes two eximerlasers, one YAG:Nd laser, and one CO₂ laser.
 11. A method for diamondcoating a substrate, the method including the steps of: forming acoating work station, by placing in proximity to a substrate supportwith an array of lasers, selected from the group consisting of eximers,YAG:Nd, and CO₂ lasers, connecting said lasers to a central processingunit, and additionally placing with proximity to said substrate supportan array of gas jets; selecting a substrate to be coated; placing saidselected substrate on said substrate support; delivering at least onegas, from said gas jet substantially to said substrate; activating saidarray of lasers; and continuing the delivery of gas and laser energyuntil a diamond coating is formed on said substrate.
 12. The method ofdiamond coating a substrate of claim 11, wherein said lasers includefour lasers.
 13. The method of diamond coating, a substrate of claim 12,wherein said four lasers include two eximer lasers, are YAG:Nd laser,and one CO₂ laser.
 14. The method of diamond coating a substrate ofclaim 11, further includes a source of N₂ gas, said source beingconnected to at least one gas jet of said array of gas jets.
 15. Themethod of diamond coating a substrate of claim 11, further including asource of CO₂ gas, said source being connected to at least one gas jetof said array of gas jets.
 16. The method of diamond coating a substrateof claim 11, wherein said selected substrate is substantially composedof WC—Co.
 17. The method of diamond coating a substrate of claim 11,wherein said lasers are operated in their pulsing modes.
 18. The methodof diamond coating a substrate of claim 11, wherein the process ofdepositing a diamond coating is abut 40 seconds.
 19. The method ofdiamond coating a substrate of claim 11, wherein the diamond growth rateduring the deposition process approaches 1 to m/s.
 20. The method ofdiamond coating a substitute of claim 11, wherein the deposition processis undertaken with an open-air environment.